The art of marking road surfaces is widely known. Pavement markings, most commonly traffic lines, can be painted on the road surface, or formed thereon by applying molten material thereto, or provided thereon by applying and adhesively securing manufactured marking tapes.
The formed or applied traffic lines or other markings thereafter form a part of the road surface and are correspondingly subjected to the wear and destructive action of traffic.
A continuing goal in the pavement marking industry is to find economical products from which to form traffic control stripes having a longer useful life than the commonly used painted stripes. The inability to achieve this goal is in part signified by the variety of products presently used to form stripes on a roadway.
One class of products comprises epoxy resin-based paints. These products have a longer life than some other paints, but nevertheless have achieved only a small usage, probably because the epoxy resin cures slowly, necessitating elaborate and expensive application procedures. Also, the applied lines tend to spall and crack, show little impact resistance, and discolor with age.
Thicker coatings, such as thermoplastic polymers extruded or sprayed while in a molten condition, have produced some increase in life because of the greater amount of material to be worn away. However, the increased amount of material also increases the cost of the markings, and both expensive equipment and uncomfortable procedures are required to apply them. Also, the high profile of these markings can be disturbing to passing traffic, and the lines are especially susceptible to removal by snowplow blades. The markings will also spall, especially when applied to concrete, apparently because of the mismatch of thermal expansion characteristics between the rigid, thick markings and the concrete.
Road markings consisting of preformed tape or strip materials are well known in the art to be advantageous in comparison to the conventional traffic markings described above. The preformed marking tapes are typically formed from a composite structure comprising a support base of a calendered rubber compound, an adhesive bottom layer, and a top-coat anti-wear layer incorporating anti-skid material and light-retroreflective elements. Such a composite structure is disclosed in many patents, such as U.S. Pat. Nos. 3,782,843 (Eigenmann), 3,935,365 (Eigenmann), 3,399,607 (Eigenmann), 4,020,211 (Eigenmann), 4,117,192 (Jorgensen), and 4,990,024 (Eigenmann).
However, this composite structure still shows less than desired durability, especially under heavy traffic conditions and high working temperatures. Marking tapes comprising a support base of unvulcanized elastomer and a top-coat of polyurethane having a high capacity to be deformed, high permanent set, and low elastic return are disclosed in the art as suitable materials to obtain a superior durability. These materials deform readily into intimate contact with irregular pavement surfaces, absorb the energy of wheel impacts without fracture and avoid the stretch-return action that has been found to loosen marking tapes from a roadway pavement. A typical example of such a marking tape can be found in U.S. Pat. No. 4,117,192 (Jorgensen).
The support bases of prior art marking tapes typically comprise a Calendered tape of an unvulcanized rubber composition. Particularly useful materials are unvulcanized compositions comprising acrylonitrile-butadiene rubber (NBR) and having good conformability and physical properties. The conformability is typically further promoted by the inclusion of extender resins such as chlorinated paraffins, hydrocarbon resins or polystyrenes. The composition can also include mineral fillers and pigments. Support base thicknesses of 1 to 1.5 millimeters (mm) are necessary to achieve desired conformability and strength in prior art marking tapes. These marking tapes generally exhibit a tensile strength ranging from 15 to 35 kilograms/centimeter.sup.2 (kg/cm.sup.2) at room temperature but at temperatures higher than 30.degree. C. typically have significantly less desirable mechanical characteristics. At the same time, vulcanized compositions cannot satisfy the need for good conformability.
Various modifications to the molecular compositions of polyurethane resins have been performed to improve conformability and elongation and to reduce elastic return. U.S. Pat. No. 4,248,932 (Tung et al.) discloses a marking tape comprising a conformable support layer and a flexible polyurethane top-coat layer. The European Patent Application having Publication No. 162,229 (Eigenmann) discloses a method to realize a conformable polyurethane top-coat by introducing into the polymer some particular molecular structures. In particular, a conformable polyurethane sheet is obtained by introducing into the polyurethane chain a deformable structure consisting of a) polyols having a functionality higher than two whose reactive hydroxyl groups are partially reacted with monofunctional compounds such as monoisocyanates, monohydroxyl derivatives and monocarboxylic acids to develop non-reactive pendant polymer branches and reduce the functionality of the polyols, and b) chain extenders, preferably aromatic materials which are sterically hindered like bisphenol-A-derivatives.
The partially reacted polyols and chain extenders improve conformability and reduce elastic return, but partially reacted polyols have a detrimental effect on abrasion resistance and on mechanical properties of the final product. In particular, the top-coat layer shows very good conformability, high elongation, and high flexibility, but also shows low mechanical properties, such as a low 10% modulus, low tensile strength and low toughness The term "10% modulus" as used herein, means the force per unit area (expressed in kg/cm.sup.2) applied to the marking tape to produce a 10% elongation relative to its initial length. These factors reduce the modulus of the marking tape, making it too soft, particularly at temperatures higher than 30.degree. C. Moreover, the use of bisphenol A or other aromatic chain extenders significantly reduces the UV light resistance of the marking tape.
These negative aspects are particularly relevant in the summer (during which pavement temperatures can increase to over 50.degree. C.) and in heavy traffic conditions, e.g., at street intersections where vehicles are accelerating, braking and turning, and applying a very powerful thrust to the tape. This frequently causes damage to the marking tape, which locally flakes off, wrinkles up, and is sometimes torn apart. Moreover, these thrusts tend to displace the tape in the direction of the force, i.e., cause the tape to slide on the road pavement, thus detaching the tape from the pavement. Another negative aspect of this conformable marking tape is that it is prone to picking up dirt (i.e., dust, sand, pebbles or gravel, and the like) at temperatures over 30.degree. C. In practice, this marking tape exhibits a very high Conformability to pavement but has too short of a life span.
U.S. Pat. No. Re. 31,669 (Eigenmann), a reissue of U.S. Pat. No. 4,146,635, discloses the use of a non-woven material interposed between a support base and a polyurethane top-coat to obtain a stiffer, less deformable and less temperature-sensitive marking tape. However, such a construction tends to have a high elastic return and a low adhesive strength, both of which promote detachment of the tape from the roadway.
Accordingly, in Spite of much work in the field of preformed marking tapes, there is still the need for an improved marking tape that exhibits a high permanent set with moderate elongation, high mechanical properties, less temperature sensitivity, and high durability under any Weather and traffic conditions.